Pancake Distribution Research Articles

Temporal Evolution of Electron Accelerations behind the Dipolarization Front in the Earth's Magnetotail

We report the temporal evolution of electron pitch angle distributions behind the dipolarization front (DF) in the Earth's magnetotail with observations of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. Taking advantage of multipoint observations from the THEMIS mission lined up in space, we study pitch angle distributions of energetic electrons behind the DF during two typical events. Pancake, rolling-pin, and cigar distributions are observed sequentially during the acceleration process. Based on Liouville's theorem, it is revealed that pancake distribution is dominantly formed by betatron acceleration in the early stage, and rolling-pin distribution is generated by both dominant Fermi and weak betatron acceleration in the transition stage, while cigar distribution is formed by Fermi acceleration finally. Our results provide comprehensive in situ observational evidence of the temporal evolution of electron acceleration behind the DF during propagation.

Local Transition of Electron Pitch Angle Distribution within Flux Pileup Region behind Dipolarization Front

Dipolarization fronts (DFs), earthward-propagating magnetic transients with a strong magnetic field, are important regions favorable for energetic electron acceleration in the magnetotail. The DF-driven electron acceleration usually generates coherent pitch angle distributions (PADs) inside flux pileup regions (FPRs), i.e., strong magnetic field regions behind the DFs, such as pancake, butterfly, and cigar distributions, which dominate at different tail regions and often occur separately. Here we present unique observations of electron PAD evolution inside the FPR, showing that electron PAD underwent local transition from cigar distribution, to butterfly distribution, then toward pancake distribution, forming a U-shaped distribution. During the local transition, electron perpendicular flux (relative to the local magnetic field) is anticorrelated with magnetic field strength, contrary to traditional expectation. The unexpected feature of the electron U-shaped distribution is associated with multiple physical processes at different scales, including local expansion of flux tubes and pitch angle variation near the neutral sheet. These atypical observations can advance our current understanding of electron acceleration and transport in the magnetosphere.

Energetic Electron Pitch-angle Distributions in the Martian Space Environment: Pancake

We perform a statistical investigation of the occurrence rates of energetic electron (100–500 eV) pancake pitch-angle distributions (PADs) in the Martian space environment by utilizing 6 yr of MAVEN data. In the Martian ionosphere, we find the following: (1) at the same altitude in the terminator and night regions, the occurrences rates in the center of the southern magnetic anomaly regions are very low, but at the edges of strong magnetic fields, they increase significantly; (2) the occurrence rates of energetic electron perpendicular anisotropies on the Martian dayside increase with altitude; and (3) some closed magnetic lines in the 10°S–55°S, 30°W–125°W region at 400–800 km altitude gradually become open during the rotation of Mars from duskside to dawnside, while more closed magnetic lines are produced in the 40°S–65°S, 35°E–90°E region. In the Martian induced magnetosphere, we find the following: (1) the high-energy electron perpendicular anisotropy in the magnetosheath is the most significant; (2) the occurrence rates in the southern (Z MSO ≤−1 R M) magnetosheath are higher than those in the northern (Z MSO ≥ 1 R M) magnetosheath; (3) in the region of ∣Z MSO∣ < 0.5 R M, these high-energy electron pancake PADs are mainly concentrated in the magnetosheath region with Y MSO ∈ [−1.4RM, 2R M]; (4) the occurrence rates in the dawnside (Y MSO ≤−1 R M) magnetosheath are higher than those in the duskside (Y MSO ≥ 1 R M) magnetosheath; and (5) in the region of ∣Y MSO∣ < 0.5 R M, the occurrence rates throughout the magnetosheath are very high.

In Situ Observations of Whistler-mode Waves in Magnetic Reconnection at Mars

Whistler-mode waves are one of the most important plasma waves potentially affecting the triggering and development processes of magnetic reconnection. They are widely present in the Earth’s reconnection ion diffusion region but have not yet been reported in Mars’. Based on in situ measurements from MAVEN, the Mars Atmosphere and Volatile Evolution mission, we report for the first time whistler-mode waves in the ion diffusion region at the center of the current sheet in the Martian magnetotail. Simultaneously, pancake electron distributions with high temperature anisotropy are observed. Linear instability analyses imply that these unstable electrons can trigger the observed whistler-mode waves. Such findings not only fill the gap in observations of whistler-mode waves in the magnetic reconnection at Mars but also enrich our understanding of their generation mechanism in the reconnection region at unmagnetized planets.

Diffuse auroral precipitation driven by lower-band chorus second harmonics

Diffuse aurora at the Earth’s high latitude regions is mainly caused by the low-energy (0.1–30 keV) electron precipitation which carries the major energy flux into the nightside upper atmosphere. Previous studies have demonstrated that combined scattering by the upper- and lower- band chorus waves acts as the dominant cause of diffuse auroral precipitation, but that is not necessarily the case as these two types of waves do not always occur simultaneously, with the lower-band more often. Here we report that the lower-band chorus satisfying the preferred condition can generate their second harmonics so as to trigger the diffuse auroral electron precipitation. We find that the lower-band chorus alone can only cause the precipitation of electrons greater than 4 keV, while the self-consistently generated second harmonic is weak but still able to result in the electron precipitation below 4 keV. The combined effect of those modes results in the observed pancake electron distributions and the diffuse aurora. Our results clearly demonstrate an alternative but universal mechanism of chorus-driven diffuse aurora in the Earth, which may also apply to the auroral formation in other planetary magnetospheres.

Jovian Auroral Electron Precipitation Budget—A Statistical Analysis of Diffuse, Mono‐Energetic, and Broadband Auroral Electron Distributions

AbstractRecent observations by the Juno spacecraft have shown that electrons contributing to Jupiter's main auroral emission appear to be frequently characterized by broadband electron distributions, but also less often mono‐energetic electron distributions are observed as well. In this work, we quantitatively derive the occurrence rates of the various electron distributions contributing to Jupiter's aurora. We perform a statistical analysis of electrons measured by the JEDI‐instrument within 30–1,200 keV from Juno's first 20 orbits. We determine the electron distributions, either pancake, field‐aligned, mono‐energetic, or broadband, through energy and pitch angles to associate various acceleration mechanisms. The statistical analysis shows that field‐aligned accelerated electrons at magnetic latitudes greater than 76° are observed in 87.6% ± 7.2% of the intervals time averaged over the dipole L‐shells according the main oval. Pancake distributions, indicating diffuse aurora, are prominent at smaller magnetic latitudes (&lt;76°) with an occurrence rate of 86.2% ± 9.6%. Within the field‐aligned electron distributions, we see broadband distributions 93.0% ± 3.8% of the time and a small fraction of isolated mono‐energetic distribution structures 7.0% ± 3.8% of the time. Furthermore, these occurrence statistics coincide with the findings from our energy flux statistics regarding the electron distributions. Occurrence rates thus also characterize the overall energetics of the different distribution types. This study indicates that stochastic acceleration is dominating the auroral processes in contrast to Earth where the discrete aurora is dominating.

Pitch Angle Distribution of MeV Electrons in the Magnetosphere of Jupiter

AbstractThe magnetosphere of Jupiter harbors the most extreme fluxes of MeV electrons in the solar system and therefore provides a testbed of choice to understand the origin, transport, acceleration, and loss of energetic electrons in planetary magnetospheres. Along this objective, the Pitch Angle Distribution (PAD) of energetic electrons may reveal signatures of the dominant physical processes. Here, we analyze for the first time the PAD of MeV electrons observed by the Galileo‐Energetic Particle Detector (EPD) experiment in orbit around Jupiter from 1995 to 2003. We find that the MeV electron PADs observed by the integral channels of EPD appear relatively isotropic with a flux anisotropy lower than a factor of 3. Due to the relatively large angular apertures of the EPD telescopes, the actual anisotropy may be larger than the observed one. The fine anisotropy observed by Galileo‐EPD reveals persistent pancake distributions at the M‐shell of M = 9. Outward of this distance, at M = 15 and M = 20–60, MeV electron distributions have pancake, isotropic, and scattered beam field‐aligned distributions. The scattered beam distributions can either be evidence of outward adiabatic transport or may suggest that high‐latitude auroral acceleration can transiently supply as much trapped MeV electrons to the middle magnetosphere as the inward adiabatic transport of electrons from an outer equatorial reservoir.

Formation of Pancake, Rolling Pin, and Cigar Distributions of Energetic Electrons at the Dipolarization Fronts (DFs) Driven by Magnetic Reconnection: A Two‐Dimensional Particle‐In‐Cell Simulation

AbstractA two‐dimensional particle‐in‐cell (PIC) simulation is performed to study the formation of the pitch angle distribution of energetic electrons at dipolarization fronts (DFs) driven by magnetic reconnection. The energetic electrons at DFs are originated from the lobe region, and experience a two‐step acceleration process at the reconnection x‐line and the DFs, respectively. Three kinds of pitch angle distributions commonly observed in the magnetotail, Pancake, Rolling pin, and Cigar distributions, are formed in sequence during the propagation of the DFs. In the early stage, Pancake distributions are formed through betatron acceleration. During this stage, the flux of energetic electrons with pitch angles around 0° and 180° is low because these electrons have no time to be reflected many times at the DFs to obtain sufficient Fermi acceleration. However, the electrons with pitch angles around 90° are difficult to be trapped around the DFs for a long time, and their flux saturates quickly; while the electrons with pitch angles around 0° and 180° can be trapped inside the closed field lines and they get continuous Fermi acceleration during the propagation of the DFs. Therefore, in the later stage, the flux of energetic electrons with pitch angles around 0° and 180° gradually increases and at last exceeds that of energetic electrons with pitch angles around 90°, forming Rolling pin and Cigar distributions in sequence.

An Unexpected Whistler Wave Generation Around Dipolarization Front

AbstractDipolarization front (DF)—a sharp boundary separating hot tenuous plasmas from cold dense plasmas—is followed by a strong Bz region, which is termed flux pileup region (FPR) or dipolarizing flux bundle (DFB). Using 9‐year (2001–2009) Cluster data, we show that the FPR hosts whistler waves because of the pancake distribution of electrons, whereas the DF boundary hosts lower hybrid drift waves due to the strong density and magnetic gradients statistically. Different from statistical results, we report an unexpected case: whistler waves are generated at the DF boundary rather than inside the FPR. Using Cluster data, we observed strong whistler waves at DF boundaries but no whistlers inside FPRs, although flux tubes inside the FPRs are significantly compressed and suprathermal electrons there are perpendicularly anisotropic. We calculate wave growth rates and successfully explain the generation/damping of these whistlers. We find that 1–4 keV electrons are responsible for generation/damping of these whistlers.

Statistical Characteristics of Electron Pitch Angle Distributions Inside the Magnetopasue Based on MMS Observations

AbstractElectron pitch angle distributions (PADs) are very important to understand dynamics in the Earth's magnetosphere. Using observations of the Magnetospheric Multiscale (MMS) mission, we statistically investigate the characteristics of several types of electron PADs with energies of 200 eV to 2 keV (low energy) and 2–30 keV (energetic energy) inside the dayside magnetopause (L = 8 ∼ 13). For the low (energetic) energy level, the occurrence rates of pancake, flat‐top, butterfly, isotropy, cigar, and rolling‐pin distributions are as follows: 50.34% (45.20%), 17.92% (43.41%), 3.07% (7.28%), 4.34% (1.54%), 16.20% (1.20%), and 1.80% (0.06%), respectively. The pancake PAD is mainly located at lower L‐shells with a maximum rate near the noonside, but other types of electron PADs occur at higher L‐shells. In addition, the occurrence rate of the flat‐top PAD is nearly the same for different magnetic local time. The butterfly and rolling‐pin PADs occur in the afternoon side. The cigar PAD is mostly located in both the dawnside and duskside but is very scarce in the noonside. Furthermore, the relationships between these PADs and the solar wind dynamic pressure (Pdyn) are studied. For low‐energy electrons, the occurrence rate of the pancake distribution decreases, and that of the butterfly distribution increases with the enhancement of Pdyn. However, for energetic energy level, the occurrence rate of the pancake (butterfly) PAD does not clearly decrease (increase) with the enhancement of Pdyn at L ≤ 12. The statistical results reveal the important role of Pdyn in electron dynamics inside the magnetopause.

Episodic Occurrence of Field‐Aligned Energetic Ions on the Dayside

AbstractThe tens of kiloelectron volt ions observed in the ring current region at L ~ 3–7 generally have pancake pitch angle distributions, that is, peaked at 90°. However, in this study, by using the Van Allen Probe observations on the dayside, unexpectedly, we have found that about 5% time, protons with energies of ~30 to 50 keV show two distinct populations, having an additional field‐aligned population overlapping with the original pancake population. The newly appearing field‐aligned populations have higher occurrence rates at ~12–16 magnetic local time during geomagnetically active times. In particular, we have studied eight such events in detail and found that the source regions are located around 12 to 18 magnetic local time which coincides with our statistical result. Based on the ionospheric and geosynchronous observations, it is suggested that these energetic ions with field‐aligned pitch angle distributions probably are accelerated near postnoon in association with ionospheric disturbances that are triggered by tail injections.

Periodical Dipolarization Processes in Earth's Magnetotail

AbstractUsing high‐resolution data of Magnetospheric Multiscale mission, we observed a gradual but periodical dipolarization process (DPs) in the Earth's magnetotail for the first time. These sub‐DPs, characterized by increase of Bz component and magnetic magnitude Bt, have a period of ~16 s. The electrons have a cigar distribution in the first three sub‐DPs but have a pancake distribution in the last sub‐DP, which excites two oppositely propagating whistler waves. At the trailing boundary of the last sub‐DP, Bz decreases sharply. Meanwhile, strong tailward and duskward plasma flow, intense current, ion and electron demagnetization, and positive energy conversion from the fields to the plasmas are observed at this boundary. By comparing the motion of this boundary with the plasma flow, we infer that the flow rebounds in longer period of the last DP and causes perpendicular electron acceleration. The periodical sub‐DPs and kinetic effects on the DP could improve our understanding of substorm and the magnetotail dynamics.

Evolution of Radiation Belt Electron Pitch Angle Distribution Due to Combined Scattering by Plasmaspheric Hiss and Magnetosonic Waves

AbstractBoth magnetosonic (MS) waves and plasmaspheric hiss can resonantly scatter outer radiation belt electrons, leading to various electron pitch angle distribution. Based on electron diffusion coefficients calculations and 2‐D Fokker‐Planck diffusion simulations, we perform a parametric study to quantitatively investigate the net electron scattering effect and the relative contributions of simultaneously occurring hiss and MS waves with groups of different wave amplitude combinations. It is found that the combined scattering effects are dominated by pitch angle scattering due to hiss emissions at L = 4, when their amplitude is comparable to or stronger than that of MS waves, thereby producing the butterfly, top‐hat, flat‐top, and pancake pitch angle distributions, while the butterfly distributions can evolve over a broader energy range when MS waves join the combined scattering effects. Our results demonstrate that the relative intensities of various plasma waves play an essential role in controlling the radiation belt electron dynamics.

Anisotropic Electron Distributions and Whistler Waves in a Series of the Flux Transfer Events at the Magnetopause

AbstractWith the measurements of the Magnetospheric Multiscale mission at the magnetopause, we investigated the electron distribution and the whistler waves associated with a series of six ion‐scale flux transfer events (FTEs). Based on the magnetic field signature, each FTE can be divided into the core region and the draping region. In the draping regions of the most FTEs, the low‐energy electrons displayed a bidirectional field‐aligned distribution. The medium‐energy electrons showed a field‐aligned or beam distribution in the leading part, while a pancake distribution was presented for the electrons in the trailing part of the draping region, which has never been reported previously. The close correlation between the pancake distribution and the compression of the localized magnetic field suggests that the pancake distribution may be due to the betatron acceleration. The whistler waves associated with the FTEs were observed and categorized into the lower and upper bands according to the frequency range. The lower band whistler waves propagated in variable directions and therefore could be generated locally. The trailing part of the draping region with the electron pancake distribution was considered to be one possible source region. On the contrary, the upper band whistler waves were all found in the core region and propagated antiparallel to the magnetic field and therefore originated from the same source region. The observations confirmed that the FTEs are important channels for the mass and wave transport between the magnetosheath and the inner magnetosphere, and the electron dynamics can be modified during the FTE evolution.

Image processing for pancake ice detection and size distribution computation

ABSTRACTThis paper presents a processing scheme whose aim is to provide a tool for a rapid measurement of pancake ice size distribution from aerial photographs. The test images used in this study were collected during the flights of the Twin Otter of the Naval Research Laboratory (NRL) which assisted the cruise of the research ship ‘Sikuliaq’ in carrying out an extensive study of autumn sea ice in the southern Beaufort Sea in 2015. The processing scheme is composed of the following steps: i) image enhancement, ii) non-linear support vector machine (SVM) analysis, iii) marker-controlled watershed segmentation, and iv) ice size distribution computation. The results demonstrate the usefulness of having immediate information on pancake ice size distribution for the subsequent tasks of the field campaign.

Electron Acceleration by Dipolarization Fronts and Magnetic Reconnection: A Quantitative Comparison

Abstract Suprathermal electrons with energy from tens to hundreds of keV are frequently observed in the Earth's magnetotail. The generation of such electrons is typically attributed to magnetic reconnection, dipolarization fronts (DFs), or flux transport. However, which of these contributes more to this generation remains unclear. In this study, we quantitatively compare the electron acceleration by these processes, using the Cluster data. We analyze an event detected in the midtail and find that the suprathermal electrons there are first accelerated by magnetic reconnection and transported earthward, and then further accelerated locally at the DF. The acceleration process by reconnection and transport, resulting in an isotropic pitch angle distribution, contributes ∼70% to the total flux enhancement, while the acceleration process by the DF, resulting in a pancake distribution, contributes ∼30% to the flux enhancement. The electron acceleration at the DF is primarily attributed to a local betatron process that is successfully reproduced using an analytic model. In order to better understand this phenomenon, we examine an additional 16 similar events and find that the DFs and magnetic reconnection statistically contribute 11% ∼ 38% and 62% ∼ 89% to the total flux enhancement, respectively. This study greatly improves our understanding of the electron energization process in the magnetotail.

The Characteristic Pitch Angle Distributions of 1 eV to 600 keV Protons Near the Equator Based On Van Allen Probes Observations

AbstractUnderstanding the source and loss processes of various plasma populations is greatly aided by having accurate knowledge of their pitch angle distributions (PADs). Here we statistically analyze ~1 eV to 600 keV hydrogen (H+) PADs near the geomagnetic equator in the inner magnetosphere based on Van Allen Probes measurements, to comprehensively investigate how the H+ PADs vary with different energies, magnetic local times (MLTs), L shells, and geomagnetic conditions. Our survey clearly indicates four distinct populations with different PADs: (1) a pancake distribution of the plasmaspheric H+ at low L shells except for dawn sector; (2) a bidirectional field‐aligned distribution of the warm plasma cloak; (3) pancake or isotropic distributions of ring current H+; (4) radiation belt particles show pancake, butterfly, and isotropic distributions depending on their energy, MLT, and L shell. Meanwhile, the pancake distribution of ring current H+ moves to lower energies as L shell increases, which is primarily caused by adiabatic transport. Furthermore, energetic H+ (&gt;10 keV) PADs become more isotropic following the substorm injections, indicating wave‐particle interactions. The radiation belt H+ butterfly distributions are identified in a narrow energy range of 100 &lt; E &lt; 400 keV at large L (L &gt; 5), which are less significant during quiet times and extend from dusk to dawn sector through midnight during substorms. The different PADs near the equator provide clues of the underlying physical processes that produce the dynamics of these different populations.

The influences of solar wind pressure and interplanetary magnetic field on global magnetic field and outer radiation belt electrons

AbstractUsing the Van Allen Probe in situ measured magnetic field and electron data, we examine the solar wind dynamic pressure and interplanetary magnetic field (IMF) effects on global magnetic field and outer radiation belt relativistic electrons (≥1.8 MeV). The dynamic pressure enhancements (&gt;2 nPa) cause the dayside magnetic field increase and the nightside magnetic field reduction, whereas the large southward IMFs (Bz‐IMF &lt; −2nT) mainly lead to the decrease of the nightside magnetic field. In the dayside increased magnetic field region (magnetic local time (MLT) ~ 06:00–18:00, and L &gt; 4), the pitch angles of relativistic electrons are mainly pancake distributions with a flux peak around 90° (corresponding anisotropic index A &gt; 0.1), and the higher‐energy electrons have stronger pancake distributions (the larger A), suggesting that the compression‐induced betatron accelerations enhance the dayside pancake distributions. However, in the nighttime decreased magnetic field region (MLT ~ 18:00–06:00, and L ≥ 5), the pitch angles of relativistic electrons become butterfly distributions with two flux peaks around 45° and 135° (A &lt; 0). The spatial range of the nighttime butterfly distributions is almost independent of the relativistic electron energy, but it depends on the magnetic field day‐night asymmetry and the interplanetary conditions. The dynamic pressure enhancements can make the nighttime butterfly distribution extend inward. The large southward IMFs can also lead to the azimuthal expansion of the nighttime butterfly distributions. These variations are consistent with the drift shell splitting and/or magnetopause shadowing effect.

Banded structures in electron pitch angle diffusion coefficients from resonant wave-particle interactions

Electron pitch angle (Dαα) and momentum (Dpp) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies ≤10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = ±1, ±2, … ±5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (α) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in Dαα and Dpp coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The Dpp diffusion coefficient for ECH waves is one to two orders smaller than Dαα coefficients. For chorus waves, Dpp coefficients are about an order of magnitude smaller than Dαα coefficients for the case n ≠ 0. In case of Landau resonance, the values of Dpp coefficient are generally larger than the values of Dαα coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89° and harmonic resonances n = +1, +2, and +3, whereas for whistler mode waves, the frequencies have been calculated for angle 10° and Landau resonance. Further, in ECH waves, the banded structures appear for electron energies ≥1 keV, and for whistler mode chorus waves, structures appear for energies &amp;gt;2 keV at L = 4.6 and above 200 eV for L = 6.8. The results obtained in the present work will be helpful in the study of diffusion curves and will have important consequences for diffuse aurora and pancake distributions.

Whistler mode wave generation at the edges of a magnetic dip

AbstractOn 18 May 2011, the Time History of Events and Macroscale Interactions during Substorms satellite observed whistler mode waves associated with a magnetic dip behind a dipolarization front structure in the bursty bulk flow braking region. For the first time, we find that whistler mode waves are generated at the edges of magnetic dip rather than at the center (also known as “minimum‐B‐pocket”). Detailed wave analysis indicates that the waves are likely lower and upper band whistler mode chorus. We examine electron pitch angle distributions at the edges of dip and compare them with those at the center and far outside the magnetic dip. Results confirm that the positive temperature anisotropy and pancake distributions at the edges of magnetic dip provide free energy source for growth of the whistler mode waves. We also interpret the whole physical process of how whistler mode waves generate in this event.